UCC28780: UCC28780D does not work when it starts?

Dear Lee,

Thanks for interested in TI's solution.
Base on the information you provide, I think the bug is related to the startup circuit.
Can you double check the HVG, SWS pin and depletion-mode MOSFET?
If the startup circuit design is correct, you should find out VDD pin go higher than 17.5V which is the VDD turn-on threshold.
After VDD pin reach 17.5V the output should start to regulate, VDD will charge by the AUX winding.
Your VDD only 5.1V, which mean it didn't reach the turn-on threshold. So the issue should related to the startup circuit.

Best Regards
Kevin

Dear Lee,

It is my pleasure to help you solve the issue.
So the issue is on the depletion-mode MOSFET?

Best Regards
Kevin

Dear Lee,

It seems you use the wrong component on depletion mode MOSFET.
Can you double check it?

Best Regards
Kevin

Dear Lee,

If you use BSS126 the schematic is wrong?
The schematic use P MOS.
It should be N MOS.

Best Regards
Kevin